Characteristics Of Ammonia-nitrogen Pollution And Determination Of Risk Control Targets In An Abandoned Fertilizer Production Site

Nitrogen fertilizer is of the great important part of fertilizer industry.As large quantities of fertilizer productions moving out of the city center,the high concentration of ammonia-nitrogen in the soil and groundwater of abandoned fertilizer production sites may severely impact the surrounding environment.To explore the pollution characteristics and environmental risks of ammonia-nitrogen at fertilizer production sites,in this paper,we analyze the spatial distribution characteristics of ammonia-nitrogen in the soil and groundwater of a typical abandoned fertilizer factory through site investigation.To control the groundwater ammonia-nitrogen pollution,we use MIN-3P software to design and implement different migrating scenarios with the consideration of the transforming relationship among tri-nitrogen(ammonia-nitrogen,nitrate and nitrite),inferring the future trends of the tri-nitrogen in groundwater of the investigated site.The differences of simulation results under different scenarios are analyzed.Moreover,for the control of the soil ammonia-nitrogen pollution,the control target values for soil ammonia-nitrogen are calculated by considering the protection of human health and groundwater,respectively.The calculations may provide references for risk management and for control of ammonia-nitrogen pollution in fertilizer production sites.The results showed that:(1)The concentrations of the soil ammonia-nitrogen at the site were between 0.03 and 15000 mg/kg.The abnormally high pollution areas were in the central production areas and material storage areas.The vertical distribution of ammonia-nitrogen in the soil showed a trend of gradually increasing and then decreasing with the increase of depth in the profile.At different migration depths,the maximum concentration of ammonia-nitrogen always appeared at the interface between the upper artificial fill and the undisturbed silty clay.The fine silty clay prevented the ammonia-nitrogenfrommigrating downward,andthe ammonia-nitrogen had different migration depths as a result of the change of stratigraphic texture.Furthermore,the site was subjected to heavy groundwater pollution of ammonia-nitrogen.The concentrations of ammonia-nitrogen in perched water and phreatic water were 19.10~3320and 0.03~219 mg/L,respectively,which was 57.89%and 100%above the contamination limits,respectively.According to the layout of the original factory,the ammonia-nitrogen pollution of groundwater and soil had similar spatial distribution patterns.In addition,there was a significant linear positive correlation between free ammonia and ammonia-nitrogen in the soil.The free ammonia concentration in perched water was 0~1518mg/L,accounting for 0~80%of the total ammonia nitrogen ratio,with an average ratio of 22.28%.Free ammonia concentration in phreatic water was 0~0.86 mg/L,only accounted for 0~8.71%of the total ammonia-nitrogen concentration,which illustrated that ammonia-nitrogen in the phreatic water mainly consisted of ammonium.(2)The migration speeds of groundwater tri-nitrogen were slow,and had a tendency to migrate to the lower boundary of the site while spreading to the periphery.The simulation results of ammonia-nitrogen in three scenarios were relatively consistent due to the insignificant biochemical reactions of ammonia-nitrogen in groundwater.Therefore,the attenuation of ammonia-nitrogen mainly came from the dilution of groundwater.Considering the increasing simulation complexity of Scenario 1,2,and 3,to balance the time-consuming issue and the prediction reasonableness,Scenario 1(Non-reactive transport)can meet the predicted demand of ammonia-nitrogen.Nitrate and nitrite both have a strong biochemical process in groundwater,which are manifested by a significant reduction in nitrate and a significant accumulation of nitrite.However,microbial biomass changes have little effect on nitrate simulation results,but have a greater impact on nitrite.Therefore,the results of the simulation of nitrate in Scenario 2(Reactive transport without biomass decay)can meet the predicted demand,while nitrite needs to be carried out under more conservative conditions(eg.Scenario3:Reactive transport within biomass decay).Choosing the right simulation scenario was critical to the subsequent analysis of groundwater contamination risk.In the process of groundwater pollution control,attention should be paid to the potential environmental risks caused by the accumulation of nitrite and the excessive repair of nitrate.(3)As currently only toxicity parameters of inhalation exposure route were available for ammonia-nitrogen,the control target of soil ammonia-nitrogen was calculated by using the calculation model of the non-cancer effect risk control value by considering only the inhalation exposure route in the Technical Guidelines for Risk Assessment of Contaminated Sites.Using site-specific measured the soil-water distribution coefficient(K_d),the control target value of ammonia-nitrogen in the soil was determined to be 9195 mg/kg based on an assumption of residential land use scenario.With regard to the protection of groundwater quality,the soil control target values were calculated using the three-or two-phase equilibrium coupled groundwater dilution model and the decay process within unsaturated zone.Depending on the scenario assumptions that there was a surface ponding or not,the control target values were 6203 and 818 mg/kg,respectively.Different control target values should be selected according to the different environmental conditions,and both soil and groundwater should be controlled by means of engineering measures and institutional control.